In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of terminal-to-terminal (D2D: Device-to-Device) communication is discussed as a new function in and after Release 12 (see Non Patent Document 1).
In the D2D communication, in a terminal group configured by a plurality of adjacent user terminals, direct Device-to-Device communication is performed without passing through a network. On the other hand, in cellular communication which is normal communication in a mobile communication system, user terminals make communication through a network.
In the D2D communication, since radio communication with low transmission power can be performed between adjacent user terminals, a power consumption of the user terminals and a load on the network can be reduced in comparison with the cellular communication.